1. Field of the Invention
The present invention relates to a data length compensating device for compensating lost cells, when receiving packet data composed of a plurality of cells. More particularly, it relates to a data length compensating device to overcome some drawbacks, when ATM cells are lost in ATM data including a plurality of cells per a packet.
2. Description of the Related Art
With the spread and extension of multi-media transmission, ATM, i.e., Asynchronous Transfer Mode, is employed as one of the transfer modes of Broadband ISDN (B-ISDN), in recent years. The transfer mode means a method for multiplexing, transmitting and exchanging information signal, such as image, sound, and character data, within a narrow frequency band to a broad frequency band.
In ATM, a digital signal information is divided into a plurality of fixed length blocks, each of which is so called a cell. More particularly, user data, i.e., digital signals, are made into cells, and are transmitted.
Referring now to FIG. 1, the user data will be further described. In FIG. 1, user data 120 of digital signals, which are bursted, are divided into a plurality of fixed length data field 122. A header 121 is added to each of the fixed length data fields 122 to form an ATM cell 123.
Additionally, a length of one ATM cell 123 is, for example, 53 bytes in total, of which 48 bytes are employed as a data field 122 and 5 bytes are employed as an ATM header 121. The header 121 of the 5 bytes contains an address data, etc. The ATM cell 123 is transmitted as a unit in a network.
Referring to FIG. 2, a diagram explaining a general flow of the ATM cell 123 from an ATM network to a terminal, which is a receiver. In FIG. 2, reference numerals 130 and 131 are an ATM network and an ATM receiving terminal. The ATM cell 123 transmitted from the ATM network 130 to the ATM receiving terminal 131 is an optical signal.
In the ATM receiving terminal 131, an interface circuit 132 converts the ATM cell 123, which is the optical signal, to an electrical signal. Additionally, the circuit 132 performs ATM termination alarm processing. The ATM cell 123 converted to the electrical signal is inputted to a CRC arithmetic section 133. The section 133 calculates error correction symbols to correct symbol errors.
Further, the ATM cell 123, of which symbol errors are VPI/VCI processor 134 identifies each of the individual terminals, which receives the cell data, based on a virtual identifier VPI and the virtual channel identifier VCI. Then, the cells are sent to an individual terminal, such as an identified telephone. In FIG. 2, cells are sent to one of the individual terminals 136.
Additionally, in FIG. 2, in the case where an ATM cell is lost during transmission, which is described later, the data length compensating device 135 in the ATM receiving terminal 131 detects the lost ATM cell, and inserts dummy data for the lost cell to compensate the data length of a packet.
Then, the ATM cell, to which dummy data is inserted, to compensate the data length, is inputted the above-described individual terminal 136 to perform processing in peculiar to terminals.
FIGS. 3A and 3B show a diagram for explaining operations of compensating lost cells, which are performed in the data length compensating device 135 in the receiving terminal 131, as shown in FIG. 2. FIG. 3A is an ATM cell sequence under sending data. A plurality of ATM cells 123 appear like a burst. Each of ATM cells 123 includes a header 121 and user data 122.
SN=1 to SN=7, each of which is attached to each of the user data 122, are sequence numbers of the cells. Each of the sequence numbers SN identifies the order of sent ATM cells. In FIG. 3A, the condition of losing two ATM cells, i.e., SN=4 and SN=5, is shown.
Now, when the receiving terminal 131 decodes data with the condition of losing the ATM cells, decoded data becomes shorter as the data lost ATM cells. If the data includes synchronous signals, such as image data, loss of synchronization is generated when recovered in the receiving terminal 131, because of lost cells.
Accordingly, a sequence number SN field is provided in the user data 122, and continuous sequence numbers SN are provided to the user data, as described above, to solve the inconvenience. In the receiving terminal 131, when there are discontinuous sequence numbers SN of the received ATM cells, the terminal 131 can recognize the number of the lost cells.
In the data length compensating device 135 in the receiving terminal 131, original data length can be restored by inserting dummy data, for example, all "zero", of which length is the same as that of data of the lost cell into the lost field, as shown in FIG. 3B. Therefore, it becomes possible to prevent from being out in phase, even if user data field 122 including synchronous signals is lost.
When inserting the dummy data, a buffer is required to temporally store the data while actually detecting lost cells, inserting the dummy data, and receiving some of leading data cells. Accordingly, the present inventors has proposed an idea, which can reduce the capacity of a buffer memory (Japanese Patent Examination No. Tokkai Hei 4-362825).
In the proposed idea, as shown in FIG. 4, a receiving buffer 150 and a second buffer 151 are provided for storing the received cells and for detecting the lost cells to write the number of the lost cells, respectively. Further, every time the cells are received, the received cells are written in the receiving buffer 150, and together, the number of cells, which are lost between the previously received cell and the presently received cell, is written in the second buffer 151 as the number of the lost cells.
Consequently, when reading the cells out from the receiving buffer 150, dummy data 152 is inserted into the number of lost cells written in the second buffer 151 to restore the data length before the cells are lost (refer to 153 of FIG. 4).
However, there are some case where cell data is piled per a packet unit and the packet unit is sent in ATM network 130. In this case, more particularly, when the cells are lost with lying over a bound of packets, i.e., two packets, it becomes impossible to restore a correct packet length, because data indicating the length of an original packet is not included, even by the above-described idea, which the present inventors have proposed.